NAME

std::basic_filebuf::open - std::basic_filebuf::open

Synopsis

basic_filebuf* open( const char* s, std::ios_base::openmode mode (1)
);
basic_filebuf* open( const std::string& str, (2) (since C++11)
std::ios_base::openmode mode );
basic_filebuf* open( const std::filesystem::path& p, (3) (since C++17)
std::ios_base::openmode mode );
basic_filebuf* open( const std::filesystem::path::value_type* s, (4) (since C++17)
std::ios_base::openmode mode );

If the associated file was already open (is_open() != false), returns a null pointer
right away.

Otherwise, opens the file with the given name (s
, p.c_str()
(since C++17) or str.c_str(), depending on the overload). std::ios_base::openmode
values may be written as, e.g., std::ios_base::out | std::ios_base::app.

Overload (4) is only provided if std::filesystem::path::value_type is (since C++17)
not char.

The file is opened as if by calling std::fopen with the second argument (file access
mode) determined by the result of mode & ~std::ios_base::ate as follows, open()
fails if the result is not some combination of flags shown in the table:

mode & ~std::ios_base::ate std::fopen Action if file Action if file
binary in out trunc app noreplace access already exists does not exist
(since C++23) mode
- + - - - - "r" Failure to
+ + - - - - "rb" Read from start open
- + + - - - "r+" Error
+ + + - - - "r+b"
- - + - - - "w"
- - + + - -
+ - + - - - "wb" Destroy contents Create new
+ - + + - -
- + + + - - "w+"
+ + + + - - "w+b"
- - + - - + "wx"
- - + + - +
+ - + - - + "wbx" Failure to open Create new
+ - + + - +
- + + + - + "w+x"
+ + + + - + "w+bx"
- - + - + - "a"
- - - - + -
+ - + - + - "ab"
+ - - - + - Write to end Create new
- + + - + - "a+"
- + - - + -
+ + + - + - "a+b"
+ + - - + -

If the open operation succeeds and (openmode & std::ios_base::ate) != 0 (the ate bit
is set), repositions the file position to the end of file, as if by calling
std::fseek(file, 0, SEEK_END), where file is the pointer returned by calling
std::fopen. If the repositioning fails, calls close() and returns a null pointer to
indicate failure.

Parameters

s, str, p - the file name to open; s must point to a null-terminated string
openmode - the file opening mode, a binary OR of the std::ios_base::openmode modes

Return value

this on success, a null pointer on failure.

Notes

open() is typically called through the constructor or the open() member function of
std::basic_fstream.

Example

// Run this code

#include <fstream>
#include <iostream>

int main()
{
std::string filename = "Test.b";
std::filebuf fb;

// prepare a file to read
double d = 3.14;
if (!fb.open(filename, std::ios::binary | std::ios::out))
{
std::cout << "Open file " << filename << " for write failed\n";
return 1;
}
fb.sputn(reinterpret_cast<char*>(&d), sizeof d);
fb.close();

// open file for reading
double d2 = 0.0;
if (!fb.open(filename, std::ios::binary | std::ios::in))
{
std::cout << "Open file " << filename << " for read failed\n";
return 1;
}

auto got = fb.sgetn(reinterpret_cast<char*>(&d2), sizeof d2);
if (sizeof(d2) != got)
std::cout << "Read of " << filename << " failed\n";
else
std::cout << "Read back from file: " << d2 << '\n';
}

Output:

Read back from file: 3.14

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 596 C++98 open() could not open files in append can open in append mode
mode